Microwave tube



Oct. 29, 1957 c. K. BIRDsALL 2,811,641

MICROWAVE TUBE Filed March 51, 1954 www ffl/f www nited States Patent 2,811,641.. MICROWAVETUBE@ 5 Charles K, BirdsalhVenice, Calif., assigngljyto-l-lHughesn Aircraft Company,` Culverv City., Calif,- a @maratea of,Delaw'are Application Marehh 19.5.4, Serial-1510,4 429,140.11.

4 .ClaimsL (C1-y 259736).-

The inventionnrelates to wave ,type-..tubes. andv more. particularly to :a deviceffor. de creasingt-heenormal diszv persion of the helical.slow-wavestructure` ofxatravelingewave tubewhereby the electronic tuningrangeofwav type oscillators may-,beincrease'd or thebandwidthwofvatraveling-wave amplifier tube widenedf- Traveling-wave tubes are commonly employed in--oscil v lators, modulators, and amplifiers., Travelingewavevtubes normally are .constructed of a helicallconductor/ which is disposedy within an -evacuated.envelopeabout-the pathv of the electron stream, the helix having Fan organization such that the electromagnetic wave propagated bythe I, helix .has electric field components .in thnewdiretion of? electron flowand a velocity approximately equal to, that# of the electrons of the stream. The` stream .'t'henvinter y .acts with the wave to increase its amplitude a'sfyitlisllpropa; gated bythe helix.

Ifravelingfwave tubes are conventionally incorporated', into external feedback path traveling-wave tube` oscil tors. The necessary conditions for''sustained oscillations4 in this type of oscillator are thattherebe an integral number of wavelengths around the fee-dbaclifloopvwitljrA a); gaingreater than unity. A fspecic number of"vwa've A lengths around the feedback-loop is lgf'erier` ydesi as a mode of oscillation. In order to lprovirwie'co electronic tuning, it isy necessary thatthe os ll restricted to one of time modes of oscillano y cordance with then present invention, thejvvidtli'of-thisy electronic tuning range may be substantially*,increased'jnyu making the wave transmission,pathanomalously dirsi;v 45 perslve.

Dispersion, in gen-eral,v is proportional towthe rateof change of phase velocity withfrequencyh Whenfthisvrate is negative, dispersion issaid tobe negative vo'rfnorrri When' the. feteis positive, dispersi@ is .Sattte..b.e.p tive or anomalous. It isn known that ahtraveling-K y tube helix` and anoscillatorl feedbackpath I -uahyl normally dispersive.: In thewpasnt, it ha 'been cal, if` not impossible, to ydesigna practical aylgornval h dispersive Struewre although'. .it haebeeefeeeenizedflh. t. Shellld 0Debe availebletitweuld.haveeene'flef blelltil'r 'l Whenempleyed either aethezexternal. feetlf eth, ferthetraveling-wave.transmieienPathf, Suehlfeetrueturefmay be employedvto `displaceor cancellnormaldisper'ysion in the wave transmission', p.athof a tr 2,811,641 V Patented OctrZS, .M1957 l 2. talnceftothey wave transmission path, which addition 11e#VV phaseyelocities ofwaves propagated along the path The sti'uctureieduces tl'iephase velocity .oflthe lqvve lfreqpexrlc'zy .waves more than forA the higherA Yfre.-r waves, Sopfvthat Ythe Irate of change of1phase, velue--` frequencymaybe made positive therebyeecting` anomalous dispersion.

The`v inventionl yalso has advantages attendant Iupon its employment in traveling-wave or otherwave-typey tubesr in? geriet-al;Y For example, alossy material Iisv often `employge d along va .traveling-.wave tube, helix vfor attenuating reflected waveswhich Sometimesv produce. undesirablel sel,f-QSQillation, WhenrtheY lossy material is:ernp1oye`d, Igainis not Ionly decreased but varies` withfrequency becauseof Ynonunit'ority in the phase velocity of fthe 'prof pagated; waveproducedfby interference effects of -certain.. Space chargeQWaVesv which are propagated v'by the V electronstream. Compensationrnayv be made for this nonuniformitylinphase.velocity, however, by using the'A present,invention.'` Further, the present invention may 13eemployed in. conjunction witha helicall'slow-wave" structure toprovide awave transmission path having yzero A dispersioncfor useeither ina ultra-,broadband Vtraveling'f--y Wave-.tubeoras adelay -devicethat is entirely independentA ofafrequency.V A M It. istherefore `an object ofk the invention tojvprovide, apparatus .for -use in conjunction'with a helical slow-,waveA structureto decrease the phase-velocity of apropfagated, waveand 4no malge it'sdispersion more positive.r Y

It--is-anot-her object of theinventionrto provide a wave; type Hoscillator having a substantially increased electronic" tuning range.

Still-Tanoth'er object of this inventionis toproyide an ultra-broadband traveling-wavetype tube. l

It-'is a'frther object ofthe invention'tomproyidemeans. for Idelaying microwaves otk allkfreque'nciesxbysubstan tiallyejcual periods of time.v

presentt invention, employed in a Vtravelingwave tube. connected tofunction asanelectr/onically `tuned. Oscillator.

Fig.,- 2 isthesection. 21V-2f ofvFig. 1.V

Fig`l3 shows typical phasevelocity,characteristicsin-.- volvedin theemployment ofl the, invention whichifare. exclnetoryef its eperetien Referi'i'ng'now to the drawing, thereis shown-in Fig.. land embodiment of thgdishclosed microwaveuoscillator whic comprises v a-travelingfwave `-tubeu llnincludingV kin.- putf and outputmatchingavities 12 iand. 14, respectively, with L feedbacl circuit such as, fonexample, a-coaxial r cabl'ewlvcompleting, thek connection betweencavities 12- and 1.4*- forproviding the necessary, feedback loop re` d to produce oscillations. An envelope 18,.-wl1ich esthe;v evacuated chamber o f traveling-wave -tube onsists, oft a long cylindrical structure lwhichfh'as an- Qrtionuat ,the left extremity'. as ,viewed in. the

Vltlgpinythis, enlarged-portion .there Yis located...1

trons which is directed along a predetermined path that lies on the longitudinal axis of elongated envelope 18.

A solenoid 20 is axially positioned symmetrically about the complete length of envelope 18. An appropriate direct current is maintained in solenoid 20 by means of a connection across a potential source, such asa battery 21,` so as to produce a magnetic field which may be of the order of 600 to 1000 gauss running axially along the entire length of the tube. The purpose of this magnetic field is to keep the electron stream focused or constrained while traversing the path along the longitudinal axis of envelope 18.

Electron gun 19 comprises a cathode 22 with a heating element 24, a focusing electrode 26, and an accelerating anode 28. Heater 24 is connected across a source of potential, such as battery 25, the negative terminal of which may be connected to cathode 22. Cathode 22 is, in turn, maintained at an adjustable potential of the order of from 500 to 2000 volts negative with respect to ground by means of a connection througha resistor 27 to the adjustable tap of a potentiometer 29 which is in turn connected across a battery 30, the positive terminalof which is connected to ground. A capacitor 31 is coupled directly to cathode 22 to provide a means of varying the velocity of the electron stream along the predetermined path in accordancewith a signal. Focusing electrode 26, sometimes called a Pierce electrode, is designed to have a frustro-conical shape with a surface of revolution at 67% mechanical degrees from its axis of symmetry and is maintained at a potential of zero volts with respect to cathode 22 in order to simulate the space charge effect of an infinitely large stream so as to focus the electrons emitted from cathode 22 into a solid cylindrical electron stream. Focusing electrode 26 is accordingly connected directly to cathode 22 to accomplish this result. Anode 28 is maintained at a potential sufficiently positive with respect to the potential of cathode 22 to accelerate the electrons to a desiredV velocity by means of a connection therefrom to ground.

Disposed concentrically about the electron stream path in the direction of electron flow within envelope 18 are a matching ferrule 32 connected over a lead 34 to helix 36, which is in turn connected over a lead 38 to a matching ferrule 40. Helix 36, and ferrules 32, 40 are maintained at the same potential as anode 28 by means of a suitable connection thereto. A collector electrode 42 is disposed at the right extremity of envelope 18, as viewed in the drawing, to intercept and collect the stream electrons. Collector 42 is maintained at a potential of the order of 200 volts positive with respect to the potential of ferrule 40 so as to minimize the number of secondaryl electrons. This voltage is impressed on collector 42 by a connection to the positive terminal of a battery 44, the negative terminal of which is connected to ground.

Helix 36, which serves as a slowfwave circuit for traveling-wave tube 10, preferably has an inner diameter substantially equal to the inner diameter of ferrules 32 and 40 so that the stream electrons can be made to pass as closely to helix 36 as possible without being intercepted by the latter. A material such as tungsten is suitable for making helix 36, the principal requirement being that it retain its form, especially with respect to its pitch and diameter.

As previously mentioned, helix 36 is connected to ferrules 32 and 40 by leads 34 and 38, respectively. Leads 34 and 38 are located parallel to the electric fields excited within matching cavities 12 and 1,4. Matching cavity `12 has the configuration of a rectangular toroid with a concentric collar 46 disposed about matching ferrule 32 and a slot opening coextensive with and adjacent to lead 34. An opening 48 in the' end plate of cavity 12 facing the left end of helix 36 allows the fulllength of lead 34 to be energized and, in addition, decreases the tendency of the electric field produced by the potential` ou the cavity to disturb the flow of the stream electrons.

Cavity 14 is similarly shaped, having a corresponding concentric collar 50 arranged about matching ferrule 40 and an opening 52 facing the right end of helix 36.

The coaxial cable 16 couples input cavity 12 to output cavity 14 through a band pass filter 55 to provide an ex -ternal feedback path for the oscillator. The center conductor 54 of coaxial cable 16 extends through the apertures in the end plates of cavities 12 and 14 to connect to collars 46 and 50, respectively, while the outer con ductor of cable 16 is bonded to the periphery of the apertures. Band pass filter 55 restricts the frequency of the energy feedback to input cavity 12 so as to limit oscillation to a band of frequencies coextensive with a desired mode of oscillation. Also, the electrical length of the connection between output cavity 14 and input cavity 12 should be as short as possible for maximum range through which the oscillator of the present invention can be electronically tuned. An output for the oscillator is provided by a loop 56 which extends through an aperture in cavity 14 `to couple to the electric field therein. Cavities 12 and 14 are fabricated with an inner surface composed of a highly conductive material and are broadly resonant so as not to control the frequency of oscillation, The configuration shown for cavities 12 and 14 in the drawing may provide, for example, suitable matching from helix 36 to coaxial cable 16 over a 2:1 range of frequencies extending from 2000 to 4000 megacycles.

Due to the fact that traveling-wave amplifiers are broadband amplifiers, it is difficult to obtain a proper impedance matching for all frequencies at the output. A resistive coating 58, which may be of carbon black, is applied on the outside of envelope 18 about the center turns of helix 36 for the purpose of attenuating waves which may be reflected from the output end of tube 10 because of an impedance mismatch. Resistive coating S8, which may be applied to any structure in the vicinity of its present position, is thus employed to decrease the tendency of the tube to break into oscillation by means of internal feedback. Forward waves traveling along with the stream electrons are not appreciably attenuated since they are propagated partially by the stream. It is to be noted that there are numerous methods of attenuatng an electromagnetic wave and that the method described is merely for the purpose of illustration.

Coating 58 serves an extremely useful purpose; however, there are certain inherent disadvantages in its ernployment. In the first place, its use decreases the phase velocity of a wave being propagated through it. This causes the gain of the tube to vary unfavorably with frequency because of the interference effects of space charge waves which are propagated by the electron stream which produce narrower operating frequency bands and appreciably decrease the gain of the tube. These reduced gain and .bandwidth effects may be minimized by decreasing the phase velocity of a wave within the unattenuated regions of the wave transmission path to a velocity equal to that of the wave within the attenuated region. In accordance with the present invention, equalization of the phase velocities is accomplished by the employment of two cylinders 59, 60 disposed lengthwise of and spaced about the unattenuated regions of helix 36 and a third cylinder 64 disposed concentrically about the remaining portion of helix 36 wherein the electromagnetic wave is attenuated. Cylinders 59, 60 and 64 each comprise a plurality of conductive strips 61 which are spaced uniformly about the helix 36 with a suitable dielectric binder 62 interposed'between the conductive strips 61. The diameter of cylinder 64 is larger than the diameters of cylinders 59, 60 so as to compensate for the effect of resistive coating 58 on the phase velocity.

With respect to cylinders 59, 60, it is not necessary for strips 61 to be outside envelope 18, nor is it necessary t for them or equivalent structure to be at any critical position; however, if the diameter of helix 36 is not kept small ilsl ,16.4141

called a backward wave-.ordm l A t less for other purposes. From this standpoint, it is more practicable to employtstripslon ithe outside of envelope 18. Strips 61 should .becomposed of .a highly conductive matcraltsuch actor example-,ceppgrfor silver.; Alterman thinly...StripS.:-..61.I man be replaced. by-.,a tcQntinuQus 0.11-v ductive multilar helix having a substantially smallernurnf berioturus. per unit length.. than thatof helix-,36a Equivalent mpedancemav also `be produced by a mesh .Offinsur latedffplaited 'conductors constituted .0f .-tWQ multflar sions,

Strips-,61 4 Should .usually ble-.spaced ,apart by@ .distance Substantially Shorter thanthediameter; .0L-.cylinders 59, 60-. Likevs /ise,. V their maximum cross-.sectional 4 dimension should ,also be. as .smallas-: possible consistent-.with the.

dimensions of the helix 36 and envelope 18. A cross section of cylinder SQat section 2 2 is shown in Fig. 2.

In this figure, the symbolfa is' employed to denote the outside radius of helix 36 and b is used as the inside rad-iushof'the cylinder 59 constituted of strips-61 and .di-

electricl 625 The -ratio lof -b to a 'determines` the effectiveness ofthe strips 61 with respect-'to thel extent that-the phase velocity is decreasedi Moreparticularly,

fori-employment .inthe traveling-waveA tube f oscillator of z the-present inventor, the ratio'` isless than the quantity and ,asnstnall aspracticable in order to ,produce maximum anomalousdispersion in Vthewave transmission path. In

theaforementioned tquantity,l er, is .the relative dielectric wherein. om is the mean angular frequency of oscillation for -amode having 1; wavelengths about thefeedback loop, and D is the dispersion of the wave transmission path. It is readily appa-rentthat if ,'this;idispersion is positive i.e.. anomalous, the frequency range Aw is ,considerably increased overwhat it would'beif .itwere ynegativeas is usually the case.`

To show more clear-lythe-manner'in which the phase velocityv-iszcompensated throughout thetlengthfof' the feedback path, reference is made to Fig, 3. In-this figure-.the

frequency range Aw extending from wi, to o2 and having a mean frequency wmrepresents the electronic tuning range of the oscillator for a mode of oscillation having v7 wavelengths around the feedback loop. Within the frequency range Aw, a line 70 represents the phase velocity versus frequency characteristic for the unaltered helix 36, a line 72 represents the phase Velocity versus frequency characteristic of the helix 36 with only the resistive coating 58, and a line 74 represents the desired phase velocity characteristic of the wave-transmission path of the oscillator. In the present case, the cylinders 59, 60 reduce the phase velocity throughout the unattenuated portions of helix 36 from velocities represented by line 70 to those represented by line 74 while the velocities throughout the region coextensive with resistive coating 58 are reduced 6 from vthevelocities.represented by line 72-to those..irepie-.`- sented by the same line 74. This difference in the amount.; that it is required to .reduce the phase velocity in theat-v tenuated region necessitates thegcylinder 64 being somewhat larger than the ,cylinders 59, 60.` disposed about the unattenuated regions.` In thisgmanner then/.ave transrnils: y sionA path is maintained uniform, throughout the wave transmission path and at the same time, the dispersion of',l the path is made anomalous.

In the operation of .the oscillator,` oscillations are restricted to a single mode by band passfltervSS. The fre-.7., quency of the oscillations are tuned through this mode by y varying lthe velocity ofthe electron stream along'thewave transmission path.. The velocity of the electron vstreamV` is preferably varied by changingthe potential in'ipressed` on cathode 22 of electron gun 20 Aspreviously pointed p out, the ifrequency range through vwhich the oscillator mayl be electronically tuned in this manner is substantiallyin-v creased by making the wave transmission,pathjlanorna-H lou'sly dispersive in accordance with the present.invention.'

In order to explain more `clearly the operation .ofthen present invention, an analogy maybe made to a conventional transmission line where the phase velocity of a prop-Vv agated wave varies inversely as thepsquare root of the` product of the series inductance and shunt capacitance perV unit length of the line. In thisrespect, the conductive f conductors at right angles to eachother.

strips 6l have no substantial effect on the series inductance` of the helix 36 in that they are disposed lengthwise therewith, noting that there is no inductive coupling between Conductive l strips 61 do, however, substantially increase the ,effective Vshunt capacitance of the helix ,'36 toL effect 'a decrease inthe phase velocity of a wave propagated by thehelix 36 in accordance with the above relation',

As the transit time required for` current flow along y' stripsv61` is a finite length vof time, theireffect with respect to increasing the shunt capacitance of helix 36 increases'as the frequency of the propagated wave is decreased. ThisA effect is particularly vnoticeable where the period 'of Athe prop-agated Wave ,is comparable to this transit time.

It is apparentffrom the structure shown in Fig. l and*A the foregoing teachings that the apparatus ofthe present invention may be incorporated with any traveling-wave tube having an attenuating regional'ong a portion of the wave transmission pathtoreduce the phase Velocity along the remaining` portion of the path to make the phase velocity uniform for the entirelength of the pathf'In this case, the ratio of the diameter of the cylinder con-A stituting conductive strips 61 and dielectric material 62to the diameter ofthe helix 36 would be a function of thel characteristics of the resistive coating (i. e. the -extent to which the electric fields external to the helixare attenuated) andthe dielectric constant of the materialdntera posed between the helix 36 and the cylinder. Inthis l respect, the proper ratio is a matter of design to'be deter- 'l mined by the particular characteristics of the tube".

In addition to the above, the present inventionmay also.r be employed to provide a delay linerfor delayingtavwide, range of frequencies by a constant periodrof'tirne.` A delay line of this type could be incorporated into a travel-TV ing-wave tube to provide an ultra wide-bandtraveling wave microwave amplifier tube. This ultra wide-band amplifier is obviously realized because waves of the entire band of frequencies are all propagated at the same velocity in synchronism with the electron stream velocity so as to effect substantially uniform gain at any frequency within the band.

This constant time delay structure is provided by employing a cylinder 59 constituting a plurality of conductive strips 61 disposed uniformly about the helix 36 lengthwise along the entire length of the delayr path in the same manner as shown in Figs. 1 and 2. In this case, however, the ratio of the radii of. Fig. 2 is made approximately equal to the wherein er is the relative dielectric constant of the intervening medium between the helix 36 and the conductive strips 61.

What is claimed as new` is:

1. In a traveling-wave tube having a conductive helix for propagating electromagnetic waves along a path whereby the higher frequency components of said waves are propagated along said path at velocities less than the lower frequency components, and having electron beam means for projecting a beam of electrons in interacting relationwith the helix, means comprising a plurality of longitudinal conductors disposed lengthwise of and spaced uniformly about said helix to cause the lower frequency components of said waves to be propagated along said path at velocities less than the higher frequency components, and a medium having a predetermined relative dielectric constant, el., different from that of a vacuum, interposed between said helix and said plurality of longitudinal conductors, the ratio of the inside diameter of the cylinder formed by said plurality of longitudinal conductors to the outside diameter of said helix being less than the quantity (aa-m) 2. A broadband travelingawave amplifier tube including a conductive helix for propagating an electromagnetic signal wave over a wave transmission path, electron beam means for projecting a beam of electrons in interacting relation with said helix, a plurality of longitudinal conductors disposed lengthwise of and spaced uniformly about said helix, and a medium having a predetermined relative dielectric constant, el., interposed between said helix and said plurality of longitudinal conductors, the ratio of the inside diameter of the cylinder formed by said longitudinal conductors to the outside diameter of said helix being substantially equal to whereby all the frequency components of said signal wave are propagated along said path at substantially the same velocity.

3. A traveling-wave tube delay line comprising electron beam producing means for providing an electron stream, a conductive helix disposed about said stream in energy exchange relation therewith for propagating an electromagnetic signal wave over a wave transmission path. a plurality of longitudinal conductors disposed lengthwise of and spaced uniformly about said helix and spaced from each other by a distance which is very small relative to the diameter of said helix, and a medium of predetermined dielectric constant, el., interposed between said helix and said plurality of longitudinal conductors, the ratio of the inside diameter of the cylinder formed by said longitudinal conductors to the outside diameter of said helixbeing substantially equal to whereby all the frequency components of said signal wave are propagated along said path at substantially the same velocity.

4. A microwave oscillator comprising a conductive helix having an input circuit and an output circuit at opposite extremities thereof, a plurality of longitudinal conductors disposed lengthwise of and spaced uniformly about said helix, a medium having a predetermined dielectric constant, `el., interposed between said helix and said plurality of longitudinal conductors, the ratio of the inside diameter of the cylinder'formed by said longitudinal conductors to the outside diameter of said helix being less than to effect an anomalous dispersion, D, in said helix, means for producing an electron stream, and means for directing said electron stream contiguously along the length of said helix; a tilter having an angular frequency passband coextensive with a predetermined tuning range, Aw, having a mean frequency, wm; and means including said filter coupled from said output circuit to said input circuit to provide a feedback loop to restrict the operation of said oscillator to a single mode of oscillation coextensive with said predetermined tuning range whereby an integral number, 1;, wavelengths exist about said feedback loop whence said tuning range, Aw, is approximately equal to com K1-D) whereby the velocity of said electron stream along the length of said helix may be varied to electronically tune said oscillator to a desired frequency within an unusually large tuning range.

References Cited in the tile of this patent UNITED STATES PATENTS 2,233,748 Rust Mar. 4, 1941 2,420,559 Nelson May 13, 1947 2,521,760 Starr Sept. 12, 1950 2,578,434 Lindenblad Dec. 11, 1951 2,603,773 Field July 15, 1952 2,626,371 Barnett et al. Jan. 20, 1953 2,653,270 Kompfner Sept. 22, 1953 2,660,689 Touraton et al Nov. 24, 1953 2,704,829 Clay Mar. 22, 1955 2,730,649 Dewey Jan. 10, 1956 FOREIGN PATENTS 970,096 France June 7, 1950 278,419 Switzerland Feb. 1, 1952 OTHER REFERENCES Manual: Abstract of Serial No. 22,743, published 656 O. G. 892, Nov. 18, 1952. 

